Biology Reference
In-Depth Information
O
R
4
H
H
N
N
R
1
+
NH
3
+
N
H
H
3
N
N
OH
R
5
R
3
R
2
Figure 16:
Structure of saxitoxin.
species. Besides six novel structural analogues, it is reported to produce decarbomyol STXs, (dcSTXs),
decarbamyol gonyautoxins such as dcGTX2 and dcGTX3 (Fig. 17; Mihali
et al
., 2011). The major PSTs
produced by
Aph
.
fl os-aquae
and
Aphanizomenon
sp. NH-5 have been reported to be STX and neoSTX.
In case of
A
.
circinalis
STX and GTXs and C-1 and C-2 variants are the major STXs synthesized (Baker
and Humpage, 1994; Humpage
et al
., 1994). On the other hand, 14 strains of
Aph
.
gracile
have been
reported to produce GTX5, dcSTX, STX and neoSTX (Ballot
et al
., 2010).
STX production by
C
.
raciborskii
T3 exhibited a correlation with intracellular Na
+
levels. This
was confi rmed by supplying Na
+
channel-blocking agents like amiloride and lidocaine in presence
of Na
+
. Amiloride (1 mM) lowered cellular Na
+
and STX levels whereas lidocaine (1 µM) enhanced
total cellular Na
+
and toxin levels. It was suggested that the metabolism of STX or toxin itself might
be associated with maintenance of cellular homeostasis (Pomati
et al
., 2004). Carneiro
et al
. (2009)
investigated the effects of light intensity and quality on the production of STX and neoSTX by
C.
raciborskii
in batch cultures. The production of neoSTX was higher in cultres under light/dark cycles
at 100 µmol photons m
-2
s
-1
than at 50 and 150 µmol photons m
-2
s
-1
light intensity. The rhythmicity
in the production of toxin was lost under red light. Transcription profi les of PST-producing and non-
toxic strains of
A
.
circinalis
were compared by DNA microarray approach. Exposure of
A
.
circinalis
to lidocaine hydrochloride (1µM for 2 hrs) led to the expression of certain putative toxin-strain
distinctive DNA fragments. On the other hand, there was no change in the expression of the same
genes after exposure to 1µM lidocaine in STX-producing
C
.
raciborskii
. However, exposure to lidocaine
enhanced transcription of genes involved in physiological adaptive responses and bloom formation
in cyanobacteria such as gas vesicle structural protein A and phycocyanin (Pomati
et al
., 2006).
i) Biosynthesis
:
A hypothetical biosynthesis pathway of STX consisting of nine reactions (Shimizu
et al
.,
1984) formed the basis for the identifi cation of the role of enzymes (S-adenylhomocysteine hydrolase,
methionine aminopeptidase, sulfotransferase, a Na
+
-dependent transporter, aminotransferase and
O-carbamoyl transferase) for its
in vitro
synthesis (Kellmann and Neilan, 2007). The identifi cation
of
sxtI
(that encodes O-methylcarbamoyltransferase) gene in
C
.
raciborskii
T3 as a marker gene for
biosynthesis of STX and the presence of its homologue in two other PST-producing cyanobacteria
paved the way for the sequence analysis of the entire gene cluster in
C
.
raciborskii
T3 (Kellmann
et
al
., 2008),
A
.
circinalis
AWQC131C and
Aphanizomenon
sp. NH-5 (Fig. 18; Mihali
et al
., 2009). Due to
the presence of
sxtI
gene in a truncated state in
L
.
wollei
, 400 bp amplicons of
sxtT
and
sxtH
(gene
products responsible for hydroxylation at C-12 position) were helful in identifying and further
